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Forensic Sleuthing Ties Ring Ripples to Impacts
April 1, 2011

Like forensic scientists examining fingerprints at a cosmic crime scene, scientists working with data from NASA’s New Horizons, Cassini and Galileo missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments dating back more than 10 years ago.

The ripple-producing culprit, in the case of Jupiter, was comet Shoemaker-Levy 9, whose debris cloud hurtled through the thin Jupiter ring system during a kamikaze course into the planet in July 1994. Scientists attribute Saturn’s ripples to a similar object – likely another cloud of comet debris – plunging through the inner rings in the second half of 1983. The findings are detailed in a pair of papers published online this week in the journal Science.

“What’s cool is we’re finding evidence that a planet’s rings can be affected by specific, traceable events that happened in the last 30 years, rather than a hundred million years ago,” said Matthew Hedman, a Cassini imaging team associate who is lead author on one of the papers and is based at Cornell University, Ithaca, N.Y. “The solar system is a much more dynamic place than we gave it credit for.”

The New Horizons spacecraft took the best images of Jupiter's charcoal-black rings as it approached and then looked back at Jupiter in February 2007. The top image was taken on approach, showing three well-defined lanes of gravel- to boulder-sized material composing the bulk of the rings, as well as lesser amounts of material between the rings. New Horizons snapped the lower image after it had passed Jupiter on February 28, 2007, and looked back in a direction toward the sun. The image is sharply focused, though it appears fuzzy due to the cloud of dust-sized particles enveloping the rings. The dust is brightly illuminated in the same way the dust on a dirty windshield lights up when you drive toward a "low" sun. The narrow rings are confined in their orbits by small "shepherding" moons.

From Galileo’s visit to Jupiter, scientists have known since the late 1990s about patchy patterns in the Jovian ring. But the Galileo images were a little fuzzy and scientists didn’t understand why such patterns would occur. The trail was cold until Cassini got into orbit around Saturn in 2004 and started sending back thousands of images. A 2007 paper by Hedman and colleagues first noted corrugations in Saturn’s innermost ring, dubbed the D ring.

A group including Hedman and Mark Showalter, a Cassini co-investigator based at the SETI Institute in Mountain View, Calif., then realized that the grooves in the D ring appeared to wind together more tightly over time. Playing the process backward, Hedman demonstrated that the pattern originated when something tilted the D ring off its axis by about 300 feet (100 meters) in late 1983. The scientists found the influence of Saturn’s gravity on the tilted area warped the ring into a tightening spiral.

Cassini imaging scientists got another clue when the sun shone directly along Saturn’s equator and lit the rings edge-on in August 2009. The unique lighting conditions highlighted ripples not previously seen in another part of the ring system. Whatever happened in 1983 was not a small, localized event; it was big. The collision had tilted a region more than 12,000 miles (19,000 kilometers) wide, covering both part of the D ring and the next outermost ring called the C ring. Unfortunately spacecraft were not visiting Saturn at that time and the planet was on the far side of the sun, unable to be seen by telescopes on or orbiting Earth, so whatever happened in 1983 passed unnoticed by astronomers.

Hedman and Showalter, the lead author on the second paper, began to wonder whether the long-forgotten pattern in Jupiter’s ring system might illuminate the mystery. Using Galileo images from 1996 and 2000, Showalter confirmed a similar winding spiral pattern. They applied the same math they had applied to Saturn – but now with Jupiter’s gravitational influence factored in. Unwinding the spiral pinpointed the date when Jupiter’s ring was tilted off its axis: between June and September 1994. Shoemaker-Levy plunged into the Jovian atmosphere during late July. The estimated size of the nucleus was also consistent with the amount of material needed to disturb the Jupiter’s ring.

The Galileo images also revealed a second spiral, which was calculated to have originated in 1990. Showalter said that before Pluto-bound New Horizons flew past Jupiter in 2007, scientists programmed the spacecraft’s camera to image Jupiter’s rings under conditions when they’d likely see the patterns Galileo picked up. Those images actually showed two newer ripple patterns, in addition to the fading echo of the Shoemaker-Levy impact.

“We now know that collisions into the rings are very common – a few times per decade for Jupiter and a few times per century for Saturn,” said Showalter, an associate member of the New Horizons Jupiter Encounter Science Team. “Now scientists know that the rings record these impacts like grooves in a vinyl record and we can play back their history later.”

The ripples also give scientists clues to the size of the clouds of cometary debris that hit the rings. In each of these cases, the nuclei of the comets – before they likely broke apart – was a few kilometers wide.

“Finding these fingerprints still in the rings is amazing and helps us better understand impact processes in our solar system,” said Linda Spilker, Cassini project scientist, based at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “Cassini’s long sojourn around Saturn has helped us tease out subtle clues that tell us about the history of origins.”

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute in Boulder, Colo. JPL managed the Galileo mission for NASA, and designed and built the Galileo orbiter. The New Horizons mission is led by Principal Investigator Alan Stern of Southwest Research Institute, Boulder, and managed by the Johns Hopkins Applied Physics Laboratory, Laurel, Md., for NASA's Science Mission Directorate.